Scientists say they can lower the risk of skin cancer in mice by rubbing caffeine and another common compound from tea on their skin. Mice normally develop skin cancer if they are exposed to high levels of ultraviolet-B radiation like that from the sun for 20 weeks. That is what happened to unprotected mice in Allan Conney's laboratory at Rutgers University in New Jersey.

Mr. Conney says most hairless mice daubed with compounds found in green and black tea avoided this fate. "We applied either caffeine or a compound we call EGCG, which is a substance in green tea, for a period of 18 weeks after stopping ultraviolet light administration. That inhibited the formation of skin tumors quite markedly."

The animals got an amount of ultraviolet radiation similar to people who are heavily exposed to sunlight early in life and then have reduced exposure later. The researchers found that caffeine application reduced the number of malignant tumors in one group of mice by 72 percent compared to untreated animals. The EGCG cut them back in another group 66 percent.

Cancers are cells of an organ that continue to reproduce endlessly beyond their life expectancies until they grow into a tumor. Mr. Conney and his colleagues report in the Proceedings of the National Academy of Sciences that the chemicals appear to prevent this by increasing the rate of natural cell death in the tumor regions. "The effect seems to be selective in cell killing in the tumors but not in surrounding normal skin cells," says Mr. Conney.

The mice in the study had a type of skin cancer called squamous cell carcinoma, which most commonly occurs in fair-skinned people. It is easily treated if detected early, but can be fatal if ignored.

Sunscreen lotions protect against this and other forms of cancer by blocking damaging solar radiation. But the study shows that caffeine and EGCG work after exposure occurs.

The researchers say the next step is testing caffeine applications in people at high risk for skin cancer. Mr. Conney says caffeine is the better of the two compounds, because it would be more stable in formulations.

Would drinking tea have the same protective affect? Mr. Conney says giving it orally works in mice exposed to high levels of ultraviolet radiation, but how humans would benefit is unclear. "I'm not sure that humans would be able to ingest enough tea to have an effect on skin cancer, but that's not known," he says.